Heretofore, both ammonia and ammonium salts have found varied use in the formation, sensitization, and coating preparation of photographic emulsions. For example, ammonium bromide has been used as the halide ion source at approximately equal molar concentrations to that of the soluble silver salt in the formation of photographic emulsions. Low levels of ammonium salts have been employed for modifying the size of silver halide crystals. For example, ammonium thiocyanate has been used as a silver ion ligand and Ostwald ripener. Similarly, low levels of ammonium salts, such as ammonium thiosulfate or ammonium chloroiridite and analogous noble metal complexes which are components of a redox couple, have found use in modifying the sensitivity or sensitometric response of silver halide emulsions. Furthermore, the literature discusses the use of high concentrations of ammonium salts and of amines in combination with certain synthetic polymers or crosslinking agents that were added to silver halide gelatin emulsions to control the viscosity or hardness of the photographic composition. While it is apparent from U.S. Pat. Nos. 3,232,761; 3,645,743; 3,861,924; and 4,161,407 that amines and ammonium salts in such combinations enhance desirable physical characteristics of emulsions, it is also known, for example, from U.S. Pat. No. 3,255,000, such amines diminish the contrast of photographic emulsions and, more importantly, enhance emulsion fog. Clearly, it has been recognized that ammonia, ammonium salts, and amines can effect both the physical and sensitometric characteristics of photographic silver halide emulsions.
The foregoing applications involve modifications in the preparation of silver halide materials, their sensitivity or their physical properties. There remains a need for compounds capable of stabilizing coated silver halide emulsions during their storage against changes of sensitivity and the formation of fog. The stabilized compounds needed are neither components of a redox couple which can reduce silver ions or oxidize elemental silver, nor are they effective silver ion ligands It is an undesirable property of such ligands that they substantially alter the silver potential of emulsions, even if such emulsions contain excess soluble halide ions such as 0.002M KBr.